We claim priority to GB 9,520,703 issued Oct. 10, 1995.
The present invention relates to paper coating pigments and their production and use.
In particular, the invention concerns an improved precipitated calcium carbonate product for use as a paper coating pigment, a process for preparing the same and paper coating compositions containing such pigment.
Coated paper and coated paperboard is used for a large range of products including packaging, art paper, brochures, magazines, catalogues and leaflets. Such coated paper and paperboard is required to give a range of properties, including brightness, opacity and sheet gloss, as well as printing performance.
In an effort to attain the required properties, many paper makers use small proportions of calcined clay and/or titanium dioxide (TiO2) in their coating formulations. Such additives have the advantage that they strongly scatter light, and thus give good opacity and brightness, but their drawback is their relatively high cost.
The general principle of using a precipitated calcium carbonate (PCC) to replace partly or wholly such expensive additives has been recognised before [J. N. Ishley, E. J. Osterhuber and N. Roman, 1992 TAPPI Coating Conference Proceedings, 335-348 (1992)].
Calcium carbonate can be precipitated from aqueous solution in three different principal crystal forms: the vaterite form which is thermodynamically unstable, the calcite form which is the most stable and the most abundant in nature, and the aragonite form which is metastable under normal ambient conditions of temperature and pressure, but converts to calcite at elevated temperatures.
The aragonite form crystallises as long, thin needles having a length:diameter ratio of about 10:1, but the calcite form exists in several different shapes of which the most commonly found are the rhombohedral shape in which the length and diameter of the crystals are approximately equal, and the crystals may be either aggregated or unaggregated; and the scalenohedral shape in which the crystals are like double, two-pointed pyramids having a length:diameter ratio of about 4:1, and which are generally aggregated. All these forms of calcium carbonate can be prepared by carbonation of milk of lime by suitable variation of the process conditions.
The work of Ishley et al. reported in the reference specified above refers to the use of rhombohedral calcitic PCC. The use of aragonitic PCC in paper coating has also been reported [D. B. Crawshaw, C. H. Kahn-Schneider and P. C. Clark, 1982 TAPPI Coating Conference Proceedings, 143-164 (1982); and G. Engstrom and M. Rigdahl, Nordic Pulp and Paper Research Journal, 90-101 (1992)], although this work does not refer specifically to light scattering performance.
One of the problems with aragonitic PCC, produced by the reaction of carbon dioxide with slaked lime, is that the reaction product consists of aggregates of needle shaped particles. The aggregated structure results in poor Theological  rheological behaviour and poor paper coating performance (e.g. sheet and print gloss). A similar but less pronounced problem can occur with scalenohedral PCC.
According to the present invention in a first aspect there is provided a method for the preparation of a precipitated calcium carbonate (PCC) for use as a pigment in paper coating compositions, the method comprising the steps of (a) carbonating an aqueous lime-containing medium to produce an aqueous suspension of a PCC predominantly in a selected crystal form, (b) at least partially dewatering the PCC-containing suspension; and (c) subjecting the PCC-containing suspension to comminution by high shear attrition grinding with an attrition grinding medium.
Steps (b) and (c) may be applied in either order, ie. (b) followed by (c) or alternatively (c) followed by (b). Where step (b) is applied before step (c) a dispersing agent (as described below) is likely to be required prior to application of step (c).
The dewatering step (b) is preferably carried out using a pressure filter device operating at a pressure of at least 5 MPa, preferably at least 10 MPa. Such a device may conveniently be of the known tube press type wherein a material is pressure filtered between two co-axially disposed tubular bodies. Such devices are described for example in GB 907,485 and in GB 1,240,465. In GB 907,485 for example, the tube pressure filter essentially comprises an upright annular chamber formed between two co-axially disposed tubular bodies, which chamber is divided into inner and outer non-intercommunicating compartments by an impermeable elastic sleeve, the arrangement being such that, in use, a material to be pressure filtered is introduced into the compartment formed between one side of the elastic sleeve and one of the tubular bodies, the one tubular body supporting a filter element, and a hydraulic fluid is introduced into the compartment formed between the other side of the elastic sleeve and the other tubular body so as to compress the material to be pressure filtered against the filter element.
The comminution step (c) is preferably carried out such as to dissipate in the suspension in which the PCC is formed at least 100 kilowatt hours of energy per dry tonne of PCC. The dissipated energy may be 200 kwhr  200 kWhr or more per tonne.
The grinding medium employed in step (c) may comprise one of the hard, inorganic materials well known in the grinding of particulate materials. For example, silica sand having a median particle diameter in the range from about 0.1 mm to 4 mm, eg. 0.2 mm to 2 mm, is a preferred grinding medium. The grinding medium could alternatively be aluminium oxide, zirconium oxide, hard steel or a mixture of any of these materials.
Preferably, step (a) is carried out in an known manner by carbonating a lime containing aqueous medium. Carbonation is desirably carried out using a carbon dioxide containing gas.
We have found unexpectedly that by use of the method according to the first aspect of the present invention PCC products can be formed which have improved optical properties when compared with those prepared in the conventional manner. Such products are therefore especially suitable for producing paper coatings with improved performance. Examples of such improvements are given hereinafter.
Particles obtained in a PCC produced as in step (a) in the method according to the first aspect of the present invention will comprise aggregates as described hereinbefore. We have found unexpectedly that substantial breaking down of such aggregates occurs in both steps (b) and (c) in the method according to the first aspect of the present invention. The contribution to breakdown of the aggregates by step (b) is greater when step (b) precedes step (c) and this is one of the factors which may lead an operator to choose to apply step (b) before step (c).
We have found that when the particle aggregates are broken down in steps (b) and (c) the pH of the aqueous suspension being treated rises. We believe that the reason for this is that when PCC is formed as in step (a) unconverted lime becomes entrapped in the PCC crystal aggregates. When the aggregates are broken down this free lime is released and dissolves in the host aqueous medium. The PCC produced as in the prior art or in step (a) may for example contain by weight up to 5% free lime, eg. 0.2% to 2% free lime. The pH may rise to pH11 or more after the application of the first of step (b) and step (c). Such a pH level is undesirable in the paper coating applications in which the PCC may be employed, as described hereinafter, because it is potentially harmful to machinery and to operators who have to process the suspension.
Desirably, an additional step (d) to reduce the pH of the aqueous PCC-containing suspension is applied preferably after both steps (b) and (c) have been applied although it could be applied after the first of these two steps. The additional step (d) may be applied until the pH falls to a suitable level, eg. below pH 9 preferably to or below pH 7.5. The additional step (d) may comprise further carbonation of the PCC-containing suspension. Alternatively, or in addition, a material known for use in a reducing the pH of a mineral suspension may be added. Such a material may, for example, comprise a mild mineral acid such as phosphoric acid.
In the method according to the first aspect of the present invention the aqueous suspension formed in step (a) may have a (dry weight) solids concentration of from 10% to 25%. After application of step (c) the PCC-containing suspension may have a solids concentration by weight of at least 50%, eg. greater than 65%.
Desirably, the suspension formed after step (c) is suitable for use in the formation of a paper coating composition without further dewatering.
For example, we have found that for a given predominantly aragonitic PCC produced in a manner to give a solids level of approximately 18% by weight, the properties given in Table 1 as follows can be obtained by a method embodying the first aspect of the present invention wherein step (b) follows step (a) and step (c) follows step (b).
In the method employed to obtain the PCC product whose properties are shown in Table 1 step (b) was carried out using a tube press providing a pressure of  greater than 7 MPa and step (c) was carried out using silica sand grinding using a grinding energy expenditure of 100 kwhr per dry tonne of product.
In Table 1, the particle size parameters X1 to X4 are the percentages by weight of particles in the product at the given product stage having an esd less than respectively 2 xcexcm, 1 xcexcm, 0.5 xcexcm and 0.25 xcexcm.
Thus, it can be seen from Table 1 that the combination of steps (b) and (c) applied after step (a) unexpectedly and beneficially improves the particle size distribution of the particles of the PCC product and this provides a consequential improvement in optical properties.
A dispersing agent, e.g. one of the agents specified below, may be employed during the grinding step (c). This may conveniently be applied before step (c) is begun.
The product of step (b) or step (c) (or step (d) if employed), may be formed into a dispersed aqueous suspension, by adding a dispersing agent for the PCC, e.g. in an amount of from 0.01% to 2%, e.g. 0.02% to 1%, by weight based on the dry weight of the pigment, the suspension containing at least 60%, preferably at least 70%, by weight of dry calcium carbonate and having a viscosity of not more than 500 mPaxc2x7s as measured by means of a Brookfield Viscometer at a spindle speed of 100 rpm. This dispersed suspension may then be incorporated into a paper coating composition together with an adhesive. The adhesive may be one of the adhesives known in the art and may form from 4% to 30%, eg. less than 20% by weight, of the composition, based on the dry weight of the calcium carbonate. For example the adhesive for the pigment may generally be chosen from the known materials for use in paper coating compositions, eg. the group consisting of starches, proteinaceous adhesives such as casein, and latices of, for example, styrene butadiene rubber and acrylic polymers.
According to a second aspect of the present invention there is provided, therefore, a pigment for paper coating which comprises a PCC produced by the method of the first aspect.
A preferred form of PCC which may be the PCC according to the second aspect of the present invention has a particle size distribution such that at least 70 percent by weight and desirably at least 90 percent by weight of the PCC particles have an equivalent spherical diameter (as measured by sedimentation) of less than 1 micrometer. Desirably, at least 50% by weight (based on the dry PCC weight) have an equivalent spherical diameter of less than 0.5 micrometers. A preferred product particle size distribution for the preferred PCC is one in which the particle size distribution is such that the percentage by weight of particles have  having an equivalent spherical diameter (measured by sedimentation) smaller than 1 xcexcm, 0.5 xcexcm and 0.25 xcexcm, respectively is as follows:
96 to 99 wt % less than 1 xcexcm
50 to 80 wt % less than 0.5 xcexcm
10 to 45 wt % less than 0.25 xcexcm.
Such a distribution has not been achieved for PCC products in the prior art. The usefulness of such a distribution is demonstrated hereinafter.
The selected crystal form of the PCC according to the second aspect is preferably a form which is predominantly aragonite although a form which is predominantly calcite of the scalenohedral habit or shape is also acceptable. Desirably, the length to diameter ratio of the crystals of the selected form averages at least 3:1. The process conditions during the precipitation process required generally to achieve either principally aragonitic or scalenohedral PCC are known to those skilled in the art.
A preferred form of a method according to the first aspect to produce predominantly an aragonitic PCC comprises the following steps prior to steps (b) and (c) as described above:
(i) mixing quicklime with water at a temperature not exceeding 60 C. to give an aqueous suspension containing from 0.5 to 3.0 moles of calcium hydroxide per liter of suspension under conditions such that the temperature of the suspension increases by not more than 80 Celsius degrees;
(ii) cooling the suspension of slaked lime prepared in step (a) to a temperature in the range from 30 C. to 50 C.
(iii) passing a carbon dioxide-containing gas through the cooled suspension at a rate such that not more than 0.02 moles of carbon dioxide are supplied per minute per mole of calcium hydroxide to precipitate calcium carbonate in the suspension while the temperature thereof is maintained within the range from 30 C. to 50 C. until the pH has fallen to a value within the range from 7.0 to 7.5.
The PCC form achieved in practice is unlikely to be 100% of the selected form. It is quite usual for one PCC crystal form even when predominant to be mixed with other forms. Such mixed forms will give suitably improved product properties. We prefer that at least 50% by weight, desirably at least 80% by weight of the crystals in the PCC product produced in step (a) are of the selected form.
As noted above, the PCC product of the second aspect may be dispersed in an aqueous medium using a dispersing agent to form a dispersed aqueous suspension of the PCC. According to the present invention in a third aspect, therefore, there is provided a dispersed aqueous suspension of the PCC product of the second aspect which incorporates a dispersing agent. The dispersed aqueous suspension formed preferably contains at least 60% preferably at least 70% by weight of calcium carbonate based on the dry weight of calcium carbonate present and has a viscosity of not more than 500 mPaxc2x7s as measured by a Brookfield Viscometer at a spindle speed of 100 revolutions per minute. The dispersing agent may be present in an amount of from 0.01 percent to 2.0 percent, e.g. 0.02 percent to 1.5 percent by weight based upon the dry weight of PCC present.
The dispersing agent may be selected from the dispersing agents known in the art for the dispersion of calcium carbonate. The dispersing agent may, for example, comprise a polycarboxylate which may be a homopolymer or copolymer which contains a monomer unit comprising a vinyl or olefinic group which is substituted with at least one carboxylic acid group, or a water soluble salt thereof. Examples of suitable monomers are acrylic acid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid, maleic acid, maleic anhydride, isocrotonic acid, undecylenic acic  acid, angelic acid and hydroxyacrylic acid. The number average molecular weight of the polycarboxylate dispersing agent should be not greater that 20,000, and preferably in the range from 700 to 10,000, as measured by the method of gel permeation chromatography using a low angle laser light scattering detector.
According to the present invention in a fourth aspect there is provided a pigment composition for use in paper coating which comprises a mixture of pigments one of which comprises a PCC according to the second aspect or a dispersed aqueous suspension according to the third aspect. The said PCC may comprise predominantly an aragonitic PCC or predominantly a scalenohedral PCC, the PCC having a particle size distribution such that at least 70 percent of the particles have an equivalent spherical diameter of less than 1 micrometer and at least 50 percent of the particles have an equivalent spherical diameter of less than 0.5 micrometers.
The other pigment or pigments incorporated in the mixture according to the fourth aspect may for example be selected from titanium dioxide, calcined clay, talc, calcium sulphate, kaolin clay, calcined kaolin and precipitated or ground calcium carbonate. The pigment mixture desirably includes a mixture of the product of the second aspect and a kaolin clay. Such a pigment mixture may comprise from 5 percent to 99 percent, especially 40 percent to 70 percent, by weight of the PCC product. Platey kaolin clay is especially preferred to form the pigment mixture with the PCC product optionally together with other pigment ingredients for the reasons explained hereinafter. By xe2x80x9cplateyxe2x80x9d kaolin clay is meant a kaolin clay having an aspect ratio of at least 20:1, preferably at least 30:1.
The pigment mixture may be formed by mixing aqueous suspensions of each of the required pigments to form an aqueous suspension incorporating the mixture of pigments. Such an aqueous suspension may be a dispersed aqueous suspension and the individual aqueous suspension of pigments employed to form the mixture may each incorporate a dispersing agent. The dispersing agents employed to disperse the pigments in the individual aqueous suspensions mixed together, and the concentrations of such suspensions, may be the same or different.
According to the present invention in a fifth aspect there is provided a paper coating composition which comprises an aqueous suspension of a PCC product according to the second aspect mixed together with an adhesive. The PCC product employed in the composition may be mixed with one or more pigments as described above. The adhesive may form from 4 percent to 30 percent by weight based on the total dry weight of pigment or pigments present. The adhesive may be one of the known paper coating adhesives employed in the art, e.g. chosen from the group consisting of starches, proteinaceous adhesives such as casein and latices of, for example, styrene butadiene rubbers and acrylic polymers.
The paper coating composition according to the fifth aspect may also include one or more optional additives conventionally used in paper coating compositions, eg. a thickener, e.g. in an amount of up to two percent by weight based upon the total dry weight of pigment or pigments present. The thickener may comprise one or more substances employed as thickeners in the prior art, e.g. sodium carboxymethyl cellulose or synthetic acrylic thickeners.
The paper coating composition according to the fifth aspect may be formed by mixing together an aqueous dispersed suspension according to the third aspect, optionally with one or more further aqueous dispersed suspensions containing other pigments, with the adhesive and any other optional constituents e.g. thickener, in a manner familiar to those skilled in the art.
We have found that paper coating compositions according to the fifth aspect when applied to woodfree paper or board, especially as a topcoat on a precoated base substrate, gives excellent sheet gloss, print gloss and brightness. The performance of the material when coated onto a wood containing base, especially in a light weight coating, has been surprisingly good in two respects:
(i) The sheet opacity and brightness attained have been such that, in a paper coating composition comprising a mixture of white pigments comprising 85 parts by weight kaolin, 10 parts by weight metakaolin (calcined kaolin), and 5 parts by weight TiO2, up to 10 parts of calcined kaolin plus 5 parts of TiO2 and 25 parts of kaolin can be replaced with 40 parts of the dry PCC product according to the second aspect with no deleterious effect on sheet properties.
(ii) Even more surprisingly, it has been found that a blend of material embodying the fifth aspect with a xe2x80x9cplateyxe2x80x9d kaolin clay, i.e. a kaolin clay of high particle aspect ratio (ratio of diameter of a circular platelet of equivalent area to average platelet thickness) of at least 20, can, in some cases, give a superior gloss to either pigment alone, and the sheet brightness attained with the blend is markedly greater than would be expected by interpolating from the brightnesses of the sole pigments.